Oxide gas absorbing arrangement and method

ABSTRACT

In the oxide gas absorbing arrangement disclosed in the specification, metal foils having a thickness ≦0.05 mm are coated with an oxide gas absorbing layer containing gamma aluminum oxide. One metal foil is corrugated and joined to a smooth foil and the combined foils are rolled into a cylinder to provide parallel gas passages in which exhaust gases are exposed to the oxide gas absorbing layer. To enhance absorption turbulence is created in the gas passages by twisting the cylinder to narrow and contort the individual passages.

REFERENCE TO RELATED APPLICATION

This is a divisional of copending application Ser. No. 09/252,506 filedFeb. 18, 1999.

This application is a continuation of International Application No.PCT/EP97/04306, filed Aug. 7, 1997.

BACKGROUND OF INVENTION

This invention relates to absorbing arrangements and methods fortemporarily storing oxides of nitrogen and sulfur and removing suchoxides from the exhaust gas from internal combustion engines.

As used herein, the term “absorb” includes the chemical process forstoring gases such as, for example, by conversion of barium oxide tobarium nitrate for storage of nitrogen oxide.

U.S. Pat. No. 4,755,499 discloses an arrangement for the reversiblestorage of oxides of nitrogen and sulfur, for example from motor vehicleexhaust gases, in which the absorber is regenerated by heating in areducing atmosphere. In this arrangement, a reduction of the nitrogenoxides takes place at the same time.

A storage catalyst of that type for use in motor vehicles is describedin more detail in U.S. Pat. No. 5,402,641, in which high temperaturesabove 500° C. are necessary to regenerate the absorber. Consequently,use of the storage catalyst is possible only for motor vehicles having ahigh exhaust-gas temperature, in particular for motor vehicles with anOtto engine.

In this case, however, the possibility of use is limited since, undercertain operating conditions of internal combustion engines, such asoccur for example in city traffic, the acceleration phases cause a largeemission of nitrogen oxide, but no long-lasting high temperaturecondition such as is required to regenerate the absorber, especiallywith respect to oxides of sulfur, is attained.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod and arrangement for releasably absorbing oxides of sulfur andnitrogen which overcomes disadvantages of the prior art.

Another object of the invention is to provide an absorber for nitrogenoxides and a corresponding method, suitable especially for use with fuelconsumption-optimized engines such as Diesel and/or direct injectionOtto engines, in which regeneration of the absorber is possible even atlow exhaust gas temperatures.

These and other objects of the invention are attained by providing anarrangement for absorbing oxide gases which includes an absorption layerfor absorbing oxides of nitrogen and/or sulfur and a support for theabsorption layer which has a wall thickness ≦0.16 mm. The supportpreferably has a wall thickness ≦0.1 mm and desirably ≦0.05 mm. Thesupport may be a metal sheet or foil and may be heatable by applicationof an electric current. Preferably the support is formed so as toprovide flow passages with a structure arranged to make the flowturbulent. With this arrangement, regeneration of the absorber ispossible even at very low exhaust gas temperatures such as occur forexample in the case of direct injection Diesel engines.

According to the invention, the usual gas absorbing materials may beemployed, for example as described in U.S. Pat. No. 4,755,499, and alsoin U.S. Pat. Nos. 5,402,641 and 5,362,463. A common feature of all thesestorage materials is that they have an elevated absorption temperature,while a still higher regeneration temperature is required especially forremoving the oxides of sulfur. For most storage media of this kind,temperatures in the range from 150° to 700° C., in particulartemperatures above 300° C., are required. Such temperatures commonlyoccur in motor vehicles with Otto engines, but are comparatively rarewith Diesel engines and especially in internal combustion engines havingdirect fuel injection.

The preferred NO_(x) storage materials are distinguished in that, underconditions of net oxidation, i.e., a stoichiometric excess of oxidizingagents, such as occurs in the exhaust gas during the operation, theywill store nitrogen oxides and, upon a reduction of the excess ofoxygen, may reduce them. For this purpose, the NO_(x) storage catalystsusually include a precious metal, in particular the usual precious metalcoatings for three-way catalysts. The NO_(x)-laden storage material isadvantageously regenerated in a regenerating phase at λ≦1.

Ordinarily, various reactions take place successively or simultaneouslyon the NO_(x) storage catalyst, the most important reactions being:oxidation of the NO in the exhaust gas to NO₂, storage of the NO₂ asnitrate, decomposition of the nitrate, and reduction of the re-formedNO₂ to nitrogen and oxygen.

As described above, the course of the reactions depends, among otherthings, not only on the temperature of the catalyst but also on theconcentration of the reagents at the active region of the catalyst andthe flow velocity of the gas.

According to the invention, it has now been found that, with variousfactors capable of being combined with each other, it is possible also,at little cost, to optimize the known exhaust-gas absorbers so that theymay be employed for internal combustion engines with direct injectionand for Diesel engines. For this purpose, the wall thickness of thesupporting member on which the absorption layer is applied preferablyshould be ≦160 microns, and desirably ≦140 microns and if a metalsupport is used, a wall thickness ≦50 microns, preferably ≦40 microns,and desirably ≦30 microns, and the absorber should preferably be heatedto a temperature above the temperature of the exhaust gas.

According to the invention, it has been found that, with the use ofthin-walled ceramic supports for the absorption layer, i.e. supportingmembers having a wall thickness ≦0.14 mm, not only is a more rapidtemperature rise of the absorption layer possible, but also a thickerabsorption layer may be used. This accomplishes two objectives: in thefirst place, even short periods of high-temperature operation can beutilized for regeneration since the storage layer temperature will beincreased to the required temperature more quickly, and in the secondplace, by providing a thicker absorption layer, a higher NO_(x) orSO_(x) storage capacity can be achieved so that during operation of theinternal combustion engine a longer period of time can elapse before thestorage layer must be regenerated. Consequently, despite the lessfrequent occurrence of temperature peaks in the exhaust gas ofconsumption-optimized internal combustion engines, no failure of thestorage layer resulting from exceeding its storage saturation limit willoccur.

According to the invention, absorbers having a support member made ofmetal foil are especially suitable, and the metal foil mayadvantageously be connectable to an electric power source for resistanceheating so that, even at low exhaust gas temperatures, the absorber canbe brought to the necessary regenerating temperature by passing anelectric current through the metal support. Furthermore, by using ametal support member, the gas passages which are coated with theabsorption layer may be variously shaped, so that, for example, acontrolled turbulent flow vortex of the exhaust gas in the passages canbe established.

With especial advantage, according to the invention, supports with avariety of passage segments may be used for the absorber where, forexample, an intermediate segment of the passages is modified to producea turbulent flow. This can be done, for example, by varying the passagecross-section, or by a twisting or distortion of the passages. In thisway, the support may be adapted in a controlled way for especiallyfavorable reaction conditions along the flow passages. Another specialfeature of the support, beside a possible variation in number ofpassages in the flow direction and the provision of changes of crosssection along the flow direction, is the segmentation of the supportwhere, for example, one segment with an absorption layer is disposednear the engine outlet and another segment with an absorption layer islocated somewhat farther away. Thus, even with the most variableoperating conditions, good NO_(x) purification results can be obtainedwith fuel consumption-optimized engines.

According to the invention, it has been found that the NO_(x) storagearrangement will have especially good absorption and desorptionproperties if the flow passages for the exhaust gas are distorted in anintermediate region to achieve a turbulent flow between an inlet regionand an outlet region which do not have a distorted structure to produceturbulent flow. As a simple arrangement for generating such a turbulentflow, for example, a transition from a large to a small diameter in thepassages is effective, but twisting of the entire support in anintermediate region will also serve to generate turbulence. Theespecially favorable properties resulting from such turbulent flow arepresumably achieved by a division of the individual reaction stepsrequired to reduce nitrogen oxides among the successive regions of thesupport, with a modified intermediate region affording better conditionsfor reaction than unmodified intermediate regions.

To produce especially good oxide gas conversions, the absorption layerhas an enlarged surface area, that is, a total surface area that issubstantially larger than the area of the surface of the support memberon which it is coated. For this purpose, the absorption layer provides asurface area of at least 20 m²/g, and preferably at least 40 m²/g. Also,the absorption layer preferably has a pore volume of at least 0.2 cm³/g,and desirably at least 0.4 cm³/g, a bimodal pore size distribution withboth micropores and macropores also being acceptable. This may beachieved for example by the choice of the size of the particles formingthe absorber surface, in which mixtures or specified distributions ofdifferent particle sizes are also suitable.

An especially suitable absorption material is gamma-aluminum oxidecontaining one or more elements in the group consisting of alkalimetals, alkaline-earth metals, rare earths and/or lanthanum. Thepresence of the elements copper and manganese is also suitable. Theadded elements are usually present as oxides, or else as carbonates ornitrates, the storage effects being achieved by formation ofcorresponding nitrates and sulfates, which are then converted back tooxides or carbonates under the appropriate reaction conditions. In thisway, it is possible to absorb NO_(x) and/or SO_(x) from an exhaust gascontaining at least 1% oxygen.

As described above, the absorbed substances are desorbed from thestorage catalyst layer by elevated temperatures and in a reducingatmosphere. For this purpose, it is desirable to determine the oxygenconcentration in the exhaust gas so that the oxygen concentration, or aquantity having a known relationship to the oxygen concentration, can beutilized to control the process of absorption or desorption.

Since the temperature of the absorption layer, determined directly orindirectly, is also important the same consideration is also applicableto the temperature of the exhaust gas. Thus, the absorption layertemperature may for example be determined by measuring the temperatureof the exhaust gas or of the support member. A determination oftemperature over the operating diagram of the internal combustion engineis also possible.

With the present invention, absorption layers having a thickness of atleast 50 microns, preferably at least 70 microns, and desirably at least90 microns, can be provided. These values are average layer thickness ofa cross section and should extend over preferably at least 50% anddesirably at least 80% of the total absorber. The foregoing absorbtionlayer thickness values apply to layers on ceramic substrates. Half ofthose values apply to absorbtion layers provided on metal substrates.Such high layer thickness values permit a greater storage capacitycompared to conventional absorbers, and consequently permit longerintervals between regeneration as described above.

According to the method of the invention, regeneration of the absorbtionlayer is preferably carried out when the operating conditions of theinternal combustion engine produce a correspondingly high temperature ofthe exhaust gas and hence of the absorption layer. Especiallyadvantageous, however, is a method in which supplementary heating of theabsorption layer is provided, preferably electrically. Other possibleheating procedures include ignition control measures in Otto engines,variation of lambda, lowering of lambda below 1, and addition ofsecondary air to generate exothermia on an oxidation catalyst, and/or anexhaust ignition arrangement, as well as heating the catalyst with aburner. A segmented absorber in which the segments are heated accordingto the required reaction is especially advantageous. Thus, for example,only one absorber segment located in a downstream exhaust flow directionmay be heated, especially in case of a distinct spatial separation ofthe absorber segments. Electric heating is especially advantageous inthis case, but an injection of fuel into the exhaust gas and/or a burnermay also be used. By arranging individual segments for individualreactions at different distances from the engine exhaust manifold,thermal aging of the absorber may be reduced in addition to providingthe advantage of especially favorable reaction temperatures inindividual absorber segments.

Further, by heating the absorber such as electrically or by means of aburner, a soot deposit on the absorber which would otherwise definitelyreduce the storage capacity can be burned off. For this purpose, theadjustment of the lambda in the exhaust gas flow to a value not over 1is preferably performed after burning off the carbon deposits.

Since the release and conversion of the NO_(x) from the storage layerand the release of the oxides of sulfur from the storage layer requiredifferent temperatures, higher in the case of the sulfur oxides, it isalso possible to proceed so that a desorption of the oxides of sulfur,which are present in particular as sulfate is performed at longer timeintervals or only as needed, so that the storage layer is onlyoccasionally heated to the high temperatures needed for desorption ofthe sulfur oxides. This counteracts premature aging of the storagelayer, so that an especially good long-term stability of the absorber isachieved. This procedure may be used with the absorber arrangement andmethod described above.

BRIEF DESCRIPTION OF THE DRAWING

Further objects and advantages of the invention will be apparent from areading of the following description in conjunction with theaccompanying single drawing illustration which shows schematically arepresentative embodiment of an exhaust gas absorbing arrangementaccording to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the typical embodiment of the invention shown in the drawing anabsorber 1 of oxide gas components is mounted in an exhaust line 2 of aninternal combustion engine 3 the operation of which is controlled by amotor control unit 4. The motor control unit 4 controls an injectionpump 5 delivering fuel from a tank 6 (not shown) to a fuel injectionnozzle 7. For the sake of clarity, the numerous conventional controllines for fuel and air supply and discharge and the like leading to themotor control unit 4 are not shown.

In the illustrated exhaust line 2 a three-way catalyst 8 is mountdownstream from the absorber 1 but it is also possible to locate thethree-way catalyst 8 upstream from the absorber 1.

In this embodiment, the absorber 1 is made from two metal foils one ofwhich is smooth and the other of which is corrugated and is connected tothe smooth foil by soldering at the corrugation crests. By rolling thismultilayer foil together, a cylindrical member having a plurality ofcoaxial passages is provided. In addition, the support member of theabsorber has an intermediate region 15 which is twisted about itslongitudinal axis, so that the individual passages 16 in theintermediate region are narrowed and contorted to produce turbulence inthe exhaust gases flowing through the passages. A similarturbulence-generating structure in the intermediate region may also beachieved by providing transverse corrugations in one or both of themetal foils.

The metal foil material contains a few percent of aluminum and isanodized so that a “wash coat” containing gamma-aluminum oxide willadhere better to the metal foil surface. The aluminum oxide wash coatfurther contains one or more of the elements sodium, barium, cerium andlanthanum, providing a layer on the aluminum oxide containing the salts,ie., nitrates, oxides and hydroxides of those elements. By impregnatingthe wound support foils with the wash coats and then firing, theabsorbing layer with those salts is produced. Additionally, theabsorbing layer is impregnated with a solution containing salts of theprecious metals platinum and rhodium and possibly palladium in additionto or instead of the rhodium, from which the corresponding preciousmetals are then liberated during firing. The resulting precious metalcoating provides a three-way catalyst. The oxide gas absorber isprovided with an electrical contact 9 and mounted in a housing so thatan electric current can be passed through the foils and the absorbinglayer to a grounded housing. The electrical contact 9 is connected tothe control unit 4, and a temperature sensor 10, also connected to thecontrol unit 4, is mounted inside the housing.

Upstream from the absorber 1, a broad-band lambda probe 11 inserted inthe exhaust gas pipe, provides signals which are proportional to theoxygen concentration present in the exhaust gas to the control unit 4.In addition, a fuel injector 12 is mounted upstream from the lambdaprobe 11 and is supplied with fuel on instructions from the control unit4. Between the absorber 1 and the following catalyst 8, an air injector13 is provided, receiving air from a pump 14 controlled by the controlunit 4.

The internal combustion engine 3 is a Diesel-type engine with directinjection, producing exhaust gas which normally has a large excess ofoxygen and a temperature of about 200° to 400° C. in operation of theInternal combustion engine, nitrogen oxides and oxides of sulfur presentin the exhaust are absorbed by the absorption layer of the absorber 1 inthe form of nitrates and sulfates of sodium, barium, lanthanum and thelike, while at the same time any oxidizable constituents present, mostlyhydrocarbons, are oxidized by the precious metal coating of the absorber1.

When the saturation limit of the absorbing layer in the absorber 1 isreached, or else at predetermined time intervals or in response to othercontrol parameters, such as for example a determination of NO_(x) in theexhaust following the absorber, the absorber is regenerated, i.e. freedfrom the NO_(x), incorporated for example as barium nitrate. At the sametime, oxides of sulfur, incorporated for example as barium sulfate, maybe removed as well. For this purpose, the control unit 4 determines byway of the temperature sensor 10 whether the temperature of the absorbercoating is high enough for regeneration of the absorber layer.

If the absorber coating temperature is below 500° C., the fuel injector12 injects fuel into the exhaust, which is catalytically burned with theoxygen present in the exhaust gas on the precious metal coating of theabsorber 1, raising its temperature. Alternatively and/or additionally,the metallic support for the absorber 1 can be electrically heated by aflow of current through the terminal 9. Still other arrangements forincreasing the absorber temperature, as for example inductive heating ofthe metallic support and/or a throttling of the exhaust are possible.

As soon as the absorber 1 is heated sufficiently, a rich mixture is setin the exhaust gas, i.e. by the fuel injector 12. Preferably a throttle17 is adjusted in the intake duct 16 of the combustion engine 3 by thecontrol unit 4 so that less air is supplied to the internal combustionengine 3. This decreases the proportion of oxygen in the exhaust gas sothat NO_(x) and SO_(x) are released from the absorption layer and arereduced. Termination of the regeneration may be time-controlled or elsecontrolled by detecting the loss of exothermia effect of the reaction onthe exhaust temperature. In a further modification, the regeneration maytake place as described in United U.S. Pat. No. 5,406,790 in which theexhaust gas flow is throttled ahead of the absorber and is directed tothe absorber through a by-pass.

To convert any hydrocarbons that may remain in the exhaust gas, an airinjector 13 located downstream of the absorber 1 is activated by thecontrol unit 4 during the fuel injection by the fuel injector 12. Inthis way, any hydrocarbons still remaining are oxidized to carbondioxide and water in the downstream catalyst 8.

The arrangement described above may be used with an eddy-chamber Dieselengine. For Otto engines with direct injection, the throttle 17 as wellas the fuel injector 12 may be omitted, since in that case the fuelinjector 7 in the combustion chamber of the engine can enrich theexhaust gas sufficiently. In principle, however, operation in the samemanner as the Diesel engine is also possible with an Otto engine.

Since the output of the engine 3 may be reduced during regeneration ofthe absorbing layer, the system may be arranged so that, when the engineis operating at full power, regeneration may be suppressed at least fora certain length of time.

Although the invention has been described herein with reference tospecific embodiments, many modifications and variations therein willreadily occur to those skilled in the art. Accordingly, all suchvariations and modifications are included within the intended scope ofthe invention.

I claim:
 1. A method for removing at least one nitrogen oxide (NO_(x))from the exhaust gas of an internal combustion engine, comprising thesteps of: (a) operating an internal combustion engine to produce anexhaust gas flow containing oxygen; (b) passing exhaust gas containingoxygen over an oxide gas absorbing layer on a support member having awall thickness ≦160 microns; (c) storing the NO_(x) in the absorbinglayer; (d) heating the absorbing layer to a predetermined temperature ofat least 500° C. during the operation of the engine; (e) forming anexhaust gas from the internal combustion engine which is poor in oxygenand/or an exhaust gas having a stoichiometric excess of a reducingagent; (g) desorbing the NO_(x) from the absorbing layer and reducingthe NO_(x) in the exhaust gas which is poor in oxygen or has astoichiometric excess of reducing agent while the absorbing layer is ata temperature equal to or above the predetermined temperature; (h)forming an exhaust gas flow containing oxygen; (i) terminating heatingof the absorbing layer to the predetermined temperature; and (j)repeating steps (c) through (h).
 2. A method according to claim 1wherein the support member is a ceramic member.
 3. A method according toclaim 1 wherein the support member is a metal foil.
 4. A methodaccording to claim 1 wherein the step of heating the absorbing layer iscarried out by at least one step from the group consisting of (a)injecting fuel into the exhaust gas and producing catalytic combustion,(b) altering the operating condition of the engine, (c) electricalheating of the absorbing layer and (d) heating the exhaust gas with aburner.
 5. A method according to claim 4 wherein, before the step ofheating of the absorbing layer to a predetermined temperature of atleast 500° C. during the operation of the engine, a step of determiningwhether a temperature representative of the temperature of the absorbinglayer is at or above the predetermined temperature is carried out; andif it is determined that the temperature representative of thetemperature of the absorbing layer is equal to or above thepredetermined temperature, the steps of heating the absorbing layer atleast to the predetermined temperature during the operation of theengine and terminating the heating the absorbing layer at least to apredetermined temperature during the operation of the engine areomitted.
 6. A method for removing at least one nitrogen oxide (NO_(x))from the exhaust gas of an internal combustion engine comprising thesteps of: (a) operating the engine to produce an exhaust gas flowcontaining oxygen; (b) passing exhaust gas containing oxygen over anabsorbing layer on a support member having a wall thickness ≦160microns; (c) storing the NO_(x) in the absorbing layer; (d) determiningwhether a temperature representative of the temperature of the absorbinglayer is equal to or above a predetermined temperature of at least 500°C.; (e) forming an oxygen-poor exhaust gas flow and/or an exhaust gasflow with a stoichiometric excess of reducing agent while the absorbinglayer is at a temperature equal to or above the predeterminedtemperature; (f) desorbing the NO_(x) from the absorbing layer andreducing the NO_(x) in the oxygen-poor exhaust gas and/or in the exhaustgas flow having a stoichiometric excess of reducing agent while theabsorbing layer is at a temperature equal to or above the predeterminedtemperature; (g) forming an exhaust gas containing oxygen; and (h)repeating steps (c) through (g).
 7. A method according to claim 6wherein the support member is a ceramic member.
 8. A method according toclaim 6 wherein the support member is a metal foil.
 9. A methodaccording to any one of claims 1-8 including storing at least one oxideof sulfur (SO_(x)) in the absorbing layer and desorbing the oxide ofsulfur from the absorbing layer.
 10. A method according to any one ofclaims 1-8 wherein the step of desorbing the NO_(x) from the absorbinglayer is carried out at timed intervals.
 11. A method according to anyone of claims 1-8 wherein the step of desorbing the NO_(x) from theabsorbing layer is carried out based on the NO_(x) content of theabsorbing layer.
 12. A method according to any one of claims 1-8 whereinthe absorbing layer contains gamma-aluminum oxide and at least oneelement from the group consisting of (a) alkali metals, (b)alkaline-earth metals, (c) rare earths and (d) lanthanum.
 13. A methodaccording to any one of claims 1-8 wherein the exhaust gas is passedturbulently over the absorbing layer.
 14. A method according to any oneof claims 1-8 wherein the exhaust gas is passed through a support memberhaving a plurality of parallel gas passages containing the absorbinglayer.
 15. A method according to claim 14 wherein the exhaust gas ispassed through a plurality of support members having the absorbing layerin which the plurality of support members have a different number ofpassages and/or passages with different flow diameters and/or are spacedat least 50 cm from each other.
 16. A method according to claim 14wherein the exhaust gas is passed over a support member bearing theabsorbing layer which has curved passages.